Potholes, as in Potholes Coulee, part of the recently mentioned channeled scablands of eastern Washington. This was today’s main event, hiking up the one branch of the coulee — which I’ll describe as a valley about a little more than a mile and a half from west to east, surrounded on both the north and south by 300 foot rock walls. I’ll call it a valley, but it was actually an outlet for the big ice age floods that roared from northwestern Montana into the Columbia River basin.

The picture above (click for a larger version) is the north wall of this part of the coulee, which had waterfalls every few hundred yards. The wall consists of multiple layers of basalt from lava flows that occurred over a span of millions of years. That columnar structure high atop the wall (it is actually called a colonnade) and signifies the lower layer of a flow and one that cooled slowly compared to the irregularly fractured section above (called entablature). The lighter rock below is basalt from an earlier flow that spent some time underwater when a lake filled part of this valley.

At least that’s what I’ve absorbed over the last few days. And here’s a more scientifically grounded partial explanation, if you’re interested.